Bishop Rock’s oversized effect on breakup

Ned Rozell
907-474-7468
April 24, 2026

A bicyclist rides a snowmachine trail as it winds through wind-sculpted snowdrifts on a frozen river on a sunny day. On the left, deciduous trees rise above a cutbank. On the right, farther away, a rocky bluff topped with spruce trees towers above the river. In the distance, a line of mixed forest marks a far bank.
Photo by Ned Rozell
Forest Wagner rides his fat bike near Bishop Rock, right, a pinch point on the flow of the Yukon River, on April 5, 2026.

A few weeks ago, as my friend Forest and I rode our bikes on the vast white sheet of the frozen Yukon River downstream of Galena, the river forced us into a 90-degree hard left. There, the channel suddenly necked down from being almost a mile wide to just a quarter mile.

A 300-foot outcrop known as Bishop Rock sits at this pinch point on the middle Yukon. Its name — bestowed by someone in remembrance of an Oregon missionary who was murdered there in 1885 — comes up at this time every year when people start talking about river breakup and the potential for ice-jam flooding.

Kyle Van Peursem of the Alaska Pacific River Forecast Center mentioned Bishop Rock during a recent presentation on the potential for spring floods in communities along the state’s rivers.

Though the Yukon, Kuskokwim, Koyukuk and other rivers in central and northern Alaska are all very solid and white as of this writing, that will soon change. River breakup happens when the power of the sun melts feet of snow from the landscape and rots the ice of the river that was hard as iron for so dang long.

Ice covers the center of a muddy river as it bends sharply and passes by a bluff on the right. The underside of an aircraft wing is visible in the top of the photograph and an aircraft landing wheel is visible in the bottom.
Photo courtesy of the National Weather Service
Bishop Rock juts into the Yukon River in this photo taken by a National Weather Service observer on May 12, 2024.

Predicting when breakup will occur at any of the dozens of villages along river systems is an inexact science. The most important variable is air temperature. Warmer Aprils are good, Van Peursem said, because they allow the snow and ice to melt at a more gradual rate that won’t overwhelm river channels.

The biggest driver of the dynamic breakups that flood villages is a cold April that “compresses the time to get rid of snowmelt,” he said.

Alaska villages on rivers most often flood in springtime due to ice jams. Jams happen when meltwater shoves chunks of recently broken ice sheets together.

“I think of these as like a dam in the river,” Van Peursem said. “The breakup front (a conveyor belt of ice chunks) stops, water has no place to go and piles up behind it.”

A river, marked by dark blue, runs across the center of a satellite image of the Earth's surface, seen in green. The river widens greatly upstream of a constricted point blocked by ice, seen in light blue. Puffy clouds float above the Earth's surface across parts of the image.
NASA image courtesy of LANCE MODIS Rapid Response
NASA’s Terra satellite captured this image of the flooded Yukon River on May 28, 2013. An ice jam at Bishop Rock backed water into the village of Galena, swamping much of the town with 7 feet of water for several days.

Constrictions in rivers like Bishop Rock are common places for ice jams. In 2013, a pileup at Bishop Rock swelled the river upstream like a python and flooded Galena. The same happened in 1945, when U.S. Air Force bomber pilots dropped more than 75 bombs on the ice jam in front of Bishop Rock. They failed to dislodge the mass of ice.

Bishop Rock will soon loom large in the windows of a single-engine aircraft in which Van Peursem will fly. He will monitor that portion of the Yukon River on flights from Galena as part of the Riverwatch program.

Van Peursem said the part of the Yukon he is monitoring is trending toward a dynamic breakup due to a cold April (Galena’s low temperature on April 22, 2026, was in the single digits Fahrenheit), but “hopefully we can slowly warm up as we go into May.”

A note to my readers: This, friends, is the second-to-last Alaska Science Forum I will write. After 31 years in the saddle, I am retiring from my science-writer job here at the Geophysical Institute on May 1, 2026. Though I have planned this for a while, the date sure has snuck up. I will sum up the whole adventure in my final column next week.

A map features a star indicating the location of Bishop Rock in west-central Alaska.
Illustration by UAF Geophysical Institute
A star marks the location of Bishop Rock on the Yukon River near Galena, Alaska.

And — fear not — my boss and other leaders at the Geophysical Institute are committed to continuing the Alaska Science Forum after I leave.

Since the late 1970s, the ҹɫ' Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute.